Thanks Jack.
That's good to know. It is definitely something to consider. In a distributed storage scenario we might build a dedicated pool for that and tune the pool as more capacity or performance is needed.
Regards,
Webert Lima DevOps Engineer at MAV Tecnologia *Belo Horizonte - Brasil* *IRC NICK - WebertRLZ*
On Wed, May 16, 2018 at 4:45 PM Jack ceph@jack.fr.eu.org wrote:
On 05/16/2018 09:35 PM, Webert de Souza Lima wrote:
We'll soon do benchmarks of sdbox vs mdbox over cephfs with bluestore backend. We'll have to do some some work on how to simulate user traffic, for writes and readings. That seems troublesome. I would appreciate seeing these results !
Thanks for the plugins recommendations. I'll take the change and ask you how is the SIS status? We have used it in the past and we've had some problems with it.
I am using it since Dec 2016 with mdbox, with no issue at all (I am currently using Dovecot 2.2.27-3 from Debian Stretch) The only config I use is mail_attachment_dir, the rest lies as default (mail_attachment_min_size = 128k, mail_attachment_fs = sis posix, ail_attachment_hash = %{sha1}) The backend storage is a local filesystem, and there is only one Dovecot instance
Regards,
Webert Lima DevOps Engineer at MAV Tecnologia *Belo Horizonte - Brasil* *IRC NICK - WebertRLZ*
On Wed, May 16, 2018 at 4:19 PM Jack ceph@jack.fr.eu.org wrote:
Hi,
Many (most ?) filesystems does not store multiple files on the same
block
Thus, with sdbox, every single mail (you know, that kind of mail with 10 lines in it) will eat an inode, and a block (4k here) mdbox is more compact on this way
Another difference: sdbox removes the message, mdbox does not : a single metadata update is performed, which may be packed with others if many files are deleted at once
That said, I do not have experience with dovecot + cephfs, nor have made tests for sdbox vs mdbox
However, and this is a bit out of topic, I recommend you look at the following dovecot's features (if not already done), as they are awesome and will help you a lot:
- Compression (classic, https://wiki.dovecot.org/Plugins/Zlib)
- Single-Instance-Storage (aka sis, aka "attachment deduplication" : https://www.dovecot.org/list/dovecot/2013-December/094276.html)
Regards, On 05/16/2018 08:37 PM, Webert de Souza Lima wrote:
I'm sending this message to both dovecot and ceph-users ML so please don't mind if something seems too obvious for you.
Hi,
I have a question for both dovecot and ceph lists and below I'll
explain
what's going on.
Regarding dbox format (https://wiki2.dovecot.org/MailboxFormat/dbox), when using sdbox, a new file is stored for each email message. When using mdbox, multiple messages are appended to a single file until it reaches/passes the rotate limit.
I would like to understand better how the mdbox format impacts on IO performance. I think it's generally expected that fewer larger file translate to less IO and more troughput when compared to more small files, but how does dovecot handle that with mdbox? If dovecot does flush data to storage upon each and every new email is arrived and appended to the corresponding file, would that mean that it generate the same ammount of IO as it would do with one file per message? Also, if using mdbox many messages will be appended to a said file before a new file is created. That should mean that a file descriptor is kept open for sometime by dovecot process. Using cephfs as backend, how would this impact cluster performance regarding MDS caps and inodes cached when files from thousands of users are opened and appended all over?
I would like to understand this better.
Why? We are a small Business Email Hosting provider with bare metal, self hosted systems, using dovecot for servicing mailboxes and cephfs for email storage.
We are currently working on dovecot and storage redesign to be in production ASAP. The main objective is to serve more users with better performance, high availability and scalability.
- high availability and load balancing is extremely important to us *
On our current model, we're using mdbox format with dovecot, having dovecot's INDEXes stored in a replicated pool of SSDs, and messages stored in a replicated pool of HDDs (under a Cache Tier with a pool of SSDs). All using cephfs / filestore backend.
Currently there are 3 clusters running dovecot 2.2.34 and ceph Jewel (10.2.9-4).
- ~25K users from a few thousands of domains per cluster
- ~25TB of email data per cluster
- ~70GB of dovecot INDEX [meta]data per cluster
- ~100MB of cephfs metadata per cluster
Our goal is to build a single ceph cluster for storage that could expand in capacity, be highly available and perform well enough. I know, that's what everyone wants.
Cephfs is an important choise because:
- there can be multiple mountpoints, thus multiple dovecot instances on different hosts
- the same storage backend is used for all dovecot instances
- no need of sharding domains
- dovecot is easily load balanced (with director sticking users to the same dovecot backend)
On the upcoming upgrade we intent to:
- upgrade ceph to 12.X (Luminous)
- drop the SSD Cache Tier (because it's deprecated)
- use bluestore engine
I was said on freenode/#dovecot that there are many cases where SDBOX would perform better with NFS sharing. In case of cephfs, at first, I wouldn't think that would be true because more files == more generated IO, but thinking about what I said in the beginning regarding sdbox vs mdbox that could be wrong.
Any thoughts will be highlt appreciated.
Regards,
Webert Lima DevOps Engineer at MAV Tecnologia *Belo Horizonte - Brasil* *IRC NICK - WebertRLZ*
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